using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication3
{

    class vec
    {
        public double x;
        public double y;

        public vec(double x, double y)
        {
            this.x = x;
            this.y = y;
        }

        public vec sub(vec vec1) 
        {
            return new vec(this.x - vec1.x, this.y - vec1.y);
        }

        public double magnitude()
        {
            return Math.Sqrt((this.x * this.x) + (this.y * this.y));
        }

        public vec div(double scalar)
        {
            return new vec(this.x / scalar, this.y / scalar);
        }

        public static double dot(vec vec1, vec vec2)
        {
            double ret = (vec1.x * vec2.x) + (vec1.y * vec2.y) ;
            return ret;
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
        
            vec ball1pos = new vec(110,90);
            vec ball2pos = new vec(100,100);
            vec difference = ball2pos.sub(ball1pos);
            double distance = difference.magnitude();
            vec normal = difference.div(distance);          // unit vector of pos delta

            vec ball1vel = new vec(0, 2);
            vec ball2vel = new vec(0,-3);
            vec velocityDelta = ball2vel.sub(ball1vel);
            double velmag = velocityDelta.magnitude();
            vec velUnit = velocityDelta.div(velmag);        // unit vector of vel delta
            
            double dot = vec.dot(velUnit, normal);          // dot product of unit vectors
  			System.Console.Write(dot);
        
        }
    }
}